1. Field
The present disclosure relates generally to apparatus and methods for sampling rate conversion for wireless transceivers, and more specifically to apparatus and methods for efficient and flexible sampling rate conversion that is adaptive to system performance of the wireless transceivers.
2. Background
Advanced wireless transceivers need to support various sampling rates in either a receive chain or transmit chain to accommodate different system bandwidths, or different operating bands used in such transceivers. One part of accommodating different bandwidths, such as in baseband processing of a transceiver, is through sampling rate conversion that converts one sampling rate of a signal to another sampling rate. Such conversion could be performed at the output of an analog-to-digital converter (ADC), the input of a digital-to-analog converter (DAC), or any other portions of baseband processing utilizing sampling of signals requiring conversion or adjustment of the sampling rates. Conventional sampling rate conversion (also termed herein as “resampling”) used in transceivers to accommodate different bandwidths or bands may include integer downsampling (i.e., decreasing the rate at which a signal is sampled by an integer factor) or upsampling (i.e., increasing a sampling rate of a signal by an integer factor) or fractional sampling (i.e., changing the sampling rate according to a predetermined fractional value). These conventional techniques, however, cannot achieve sampling rate conversion for any desired specific resampling ratios, but are limited to the integer or fractional values. Thus, these conventional techniques do not have the flexibility to convert sampling rates to any desired other sampling rate.
Furthermore, conventional linear interpolators, which are used in sampling rate conversion, usually provide limited image rejection especially for wideband cellular systems such as OFDMA-based systems including Long Term Evolution (LTE), LTE Advanced, WiMax (IEEE 802.16) and Ultra Mobile Broadband (UMB). That is, the spectrum of the resampled signal in a conventional interpolator may contain several unwanted images (or aliases) of the desired signal spectrum and it is desirable to reject these unwanted images, typically through filtering. High-order interpolators with high input sampling rates can achieve better image rejection. Such interpolators, however, have high operating or sampling rates that typically require multiple numbers of full multipliers, which become extremely costly for high-speed wideband wireless transceivers in terms of hardware and power consumption. Accordingly, a need exists for more efficient and flexible sampling rate conversion with high rate input sampling to ensure proper image rejection, but with less cost in terms of hardware and power consumption.